The global demands for high quality diesel are increasingly stepped up, while the demands for the fuel oils are decreasing. On the whole, the growth speed of the global demands for diesel will go beyond that on gasoline, although the area demands vary. Therefore, more and more light diesels with low cetane number produced by catalytic cracking (FCC) are being used as the harmonic component of diesel. In order to satisfy the demands for high quality diesel, it is necessary to modify the FCC light diesel, or produce high quality FCC light diesel with high output by FCC.
In the prior art, the processes for modifying the catalytic light diesel primarily include hydrogenation and alkylation. U.S. Pat. No. 5,543,036 discloses a process for modifying the FCC recycled light oil by hydrogenation. CN1289832A also discloses a process by hydrogenation for modifying the catalytically cracked diesel, comprising, under the hydrogenation conditions, passing the feedstocks in turn through the hydrotreating catalyst and the hydrocracking catalyst in single-stage series without any intermediate separation. Due to said process, the cetane number of the diesel fraction in the product is increased by 10 units as compared with the feedstocks, and the sulfur content and nitrogen content are notably decreased. U.S. Pat. No. 4,871,444 discloses a process for increasing the cetane number of a FCC recycled light oil, comprising the alkylation of the FCC recycled light oil with a linear alkylene having 3 to 9 carbon atoms in the presence of solid acid catalyst. U.S. Pat. No. 5,171,916 discloses a process for modifying a FCC recycled light oil, comprising the alkylation of the FCC recycled light oil with α-C1-4 alkylene or coker gas oil on a solid acid catalyst.
Another process of enhancing the quality of catalytic light diesel directly is achieved by varying the processing parameters or catalysts of the catalytic cracking CN1900226A discloses a catalytic cracking promoter for producing more diesel and a process for preparing the same. By the addition of a certain amount of such promoter, the diesel yield of FCC catalytic unit will be increased, and the product distribution will be improved without any change of the catalyst initially used in the refining unit. However, such process does not mention any improvement in the properties of diesel. CN1683474A also involves a catalytic cracking promoter for producing more diesel and a process for preparing the same. CN1473908A relates to a process for producing diesel from heavy oil and residue with Ca2+-EDTA catalytic cracking. CN101171063A relates to a fluidized catalytic cracking (FCC) process for improving the quality of the distillate useful as harmonic oil of a diesel fuel. The FCC process combines the stage FCC conversion with the interstage separation of the multi-cyclic arene species. The reaction zones with lower and higher severities and selective separation of molecules in the riser of the FCC reactor together increase the outputs of the distillate with high diesel quality. The process emphasizes to obtain a saturated hydrocarbon enriched diesel fraction with high cetane number by a membrane separation.
Another process to enhance the quality of the catalytic light diesel uses the duplex combination of hydrogenation and catalytic cracking. For example, CN1896192A feeds the wax oil together with the recycled catalytically cracked heavy oil and the catalytically cracked light diesel into a hydrogenation unit, and feeds the hydrogenation tail oil into a catalytic cracking unit. The process can reduce the contents of aromatics and sulfur in the diesel, and increase the cetane number thereof. CN1382776A involves a process combining the hydrogenation of residues and the catalytic cracking of heavy oils. The processes in above patents do not set requirements on the procedures of catalytic cracking, but modify the diesel by hydrogenation.
CN101362959A discloses a catalytic conversion process for producing propylene and a gasoline with high octane number, comprising contacting the raw materials difficult to be cracked with a thermo-regenerated catalyst and conducting the cracking reaction under the conditions of: a temperature between 600 and 750° C., a weight hourly space velocity between 100 and 800 h−1, a pressure between 0.10 and 1.0 MPa, a ratio of the catalyst to the raw materials between 30 and 150, a ratio of the steam to the raw materials between 0.05 and 1.0; mixing the reaction stream with the raw materials easy to be cracked and conducting the cracking reaction under the conditions of: a temperature between 450 and 620° C., a weight hourly space velocity between 0.1 and 100 h−1, a pressure between 0.10 and 1.0 MPa, a ratio of the catalyst to the raw materials between 1.0 and 30, a ratio of the steam to the raw materials between 0.05 and 1.0; separating the spent catalyst and the reaction oil vapors, followed by feeding the spent catalyst into a stripper, stripping and coke-burning the catalyst, and recycling the regenerated catalyst to the reactor; and separating the reaction oil vapors to obtain the propylene and gasoline with high octane number as the target products, as well as re-cracked raw materials. The re-cracked raw materials comprise a fraction with distillation range of from 180 to 260° C. and a raffinate of heavy aromatics. The yield and selectivity for propylene are much increased by the process, and the yield of gasoline and the octane number are also increased significantly. The yield of dry gas decreases by 80% by weight or more.